Hi Joseph,
Thanks for the great idea, and sorry it took so long to get you a response. The original file function you referenced no longer exists, and instead now aliases to wp:mw_newMediaObject.

In the current release, to add the description, what you could do is just have a plugin that hooks into xmlrpc_call_success_mw_newMediaObject and use the $id and $args parameters to set the description:

Basically, in your XMLRPC call to wp:mw_newMediaObject, add a parameter for the description in your $args array and then use a plugin that hooks into the wp:mw_newMediaObject action after uploading the attachment to save that description into the description of the attachment post.